Corinnguard
Hero
Hmm... a number of Benders seen in both series were able to manipulate secondary elements. Water- Ice (Cold), Steam (Fire), Blood (Necrotic), Healing (Radiant), Wood (Piercing, Bludgeoning, Slashing). Fire- Lightning. Air- Sound* (Thunder), Earth- Sand (Fire, Acid, or Necrotic?), Metal (Slashing, Piercing, Bludgeoning), Magma (Fire).Since this is D&D, I could see including maneuvers that deal with acid, lightning, and the other damage types as well.
*With sound propagating through the air, sound or Thunder ought to be something an Airbender should be capable of manipulating.